Mobile handling device with hydraulic circuit, which hydraulic circuit comprises a lifting. cylinder arranged in a lifting device intended for handling a variable load and an accumulator for recovering or recycling the lowering load energy.
Excavators, trucks, container handlers etc. and a large number of other mobile handling machines which are intended to handle a variable load have one or more lifting cylinders for lifting the load for which the unit is designed. The great majority of mobile handling devices used today have no energy recovery facility whatever for the lowering load, meaning that the lowering load energy, most often in connection with passage through a control valve which determines the lifting and lowering motion, is converted to heat which then has to be cooled away. The heating of the hydraulic oil to undesirable temperatures is a long familiar problem for machinery manufacturers and end customers.
As well as eliminating the heat problem, there is naturally a constant desire to be able to minimize the energy requirement when operating a mobile handling device. For mobile handling devices, for example excavators, it normally holds good that arm systems and equipment have a dead weight which is dependent on the task to be executed. The lowering load energy can thus vary considerably under different conditions. There is for example a considerable difference in the tool weight of a machine equipped only for excavation and another machine which is equipped with a rotor tilt tool attachment for carrying out associated work. Many similar examples are generally known to the expert in the field, but in short it can be said that the dead weight of the arm system can vary from around 30% to 80% of the maximum lifting force. In addition to this, there is also a great difference in the kind of work the machine is intended to carry out, for example whether levelling or loading is involved. It is widely known that an excavator can execute up to five different work tasks perfectly well in some shifts. It is previously known in connection with mobile handling machines, for example by way of WO 9311363 and DE 4438899, to provide a hydraulic circuit with recovery in connection with lifting or lowering movements, an accumulator circuit being provided which utilizes the potential energy of the arm system and the load on lowering. These known systems are based on using at least two lifting cylinders connected to one another. This is obviously an undesirable restriction, since in many applications it is desirable to use just one lifting cylinder. Certain known embodiments according to the aforementioned prior art have also caused impaired visibility. Furthermore, this known solution also involves several moving parts and in certain cases causes uneven loading to occur. A common feature of these known systems is that the energy saving systems are suitable for lifting work in which a large part of the energy supplied goes on lifting the actual arm system, which makes them poorly adapted to mobile handling machines with considerably varying loads. A further disadvantage is that one is forced to work at very high pressures, often of up to 350 bar, which results in a considerable increase in the price of small effective volumes in relation to the size. In addition, commonly known systems give rise to certain control problems which are difficult to solve. These known systems do not therefore solve the problems in an optimal manner. A first object according to the invention is therefore to solve this problems in a more optimal manner.
A specific problem in the field of the invention relates to mobile handling machines which also equipped with a slewing motion for the lifting device, of the excavator type, which are equipped with a roller path and one or more hydraulic motors which transmit the force for the slewing motion by means of toothed gearing. Such machines have a slewing part, which in relation to the load represents a large mass, signifying a large moment of inertia which has to be overcome at each new start of a slewing motion. This makes great demands if speed is desired in the slewing motion, and it is not unusual for more than 40% of the motor power to be used for starting this. During the acceleration phase, the pressure will rise to the maximum value and the flow increases until its desired rotation speed is attained, following which the pressure is reduced to the level required for overcoming the no-load losses. During the deceleration phase, the movement energy attained is then braked away in machines of this kind by throttling the return flow via overflow valves, which in addition to an energy loss gives rise to not inconsiderable heating of the hydraulic medium.
In most hydraulic systems for the purpose, the system is also set up using a counter-pressure on the xe2x80x9cmeter inxe2x80x9d side to prevent the movement racing away ahead of the xe2x80x9cmeter outxe2x80x9d flow, i.e. to avoid so-called hydraulic play. This setting up will function in principle such that one both accelerates and brakes at the same time, which of course is very disadvantageous from the energy point of view. It is not unusual to brake away 30% of the power supplied in this manner during the stewing movement itself
It is perceived that it would be advantageous if the movement energy from the slewing motion could be recovered, not least against the background of the problems already discussed above with regard to the increase in the oil temperature, which among other things has a negative effect on the life of the oil. Furthermore, it is perceived that it is a disadvantage to have to provide special auxiliary systems to ensure the requisite filling with hydraulic oil in a closed accumulator circuit system according to the prior art.
The object of the present invention is to eliminate or at least reduce the aforementioned problems, which is achieved by means of the fact that the hydraulic circuit comprises a variable hydraulic machine with two ports, which hydraulic machine is capable via a drive unit of giving off full system pressure in two flow directions to said ports, one port being connected to an accumulator and the other port being connected to a lifting cylinder.
Thanks to the use of this type of hydraulic machine in the hydraulic circuit, the oil can be pumped directly between the accumulator and the lifting cylinder, which signifies a considerable simplification and means that control losses are eliminated in effect. The invention thus not only solves the heat problem, but also signifies a substantial energy saving, which surprisingly has been shown to amount to around 30%.
It is true that a hydraulic circuit is previously known from U.S. Pat. No. 4,646,518 which comprises a variable reciprocating pump which acts together with an accumulator. This known system, however, concerns a quite specifically distinct device, namely a feed pump for crude oil, which is a stationary installation of considerable dimensions. Thus it does not relate to a mobile device or recovery of lowering load energy from a lifting device with a variable load, but only continuous recovery of the constant load which the oil pump itself brings about. The known system thus relates to a field quite specifically distinct from the present invention, which refers to mobile handling devices in which the variable load in itself represents the most important source for the recovery of energy and in which the variable load in itself is the cause of the problem of overheating of the hydraulic medium according to the prior art.
According to a further aspect according to the invention, it holds good that in communication with said hydraulic circuit is a control valve, said variable reciprocating pump being connected to at least one of said accumulator and said lifting cylinder without the connection going via the control valve, preferably both said accumulator and said lifting cylinder being connected in such a way to the reciprocating pump. A system of this kind signifies a considerable simplification, not least in control terms, and means that control losses are in effect eliminated. In addition it is the case that the control valves in existence today are not normally made to control the flow from the consumer unit to the motor port, but are designed to control the flow from the motor port to the consumer unit. This is a disadvantage from the operator viewpoint, as no pressure compensation can take place, which means that the lifting speed is influenced by the load. A system according to the invention can eliminate all these disadvantages and is also more efficient.
According to a further aspect according to the invention, the hydraulic circuit comprises a first stop valve disposed in the line between one port of the hydraulic machine and the lifting cylinder, and a second stop valve disposed in the line between the second port of the hydraulic machine and the accumulator, meaning that leakage losses, which would otherwise occur in the hydraulic machine, can be eliminated during periods when the hydraulic machine is in the neutral position, i.e. when the lifting device is not intended to execute work in a vertical direction.
According to a further aspect according to the invention, it is the case that said first and second stop valves are controlled by a servo circuit which comprises a servo pump and a valve unit, by means of which the stop valves are actuated into the open position when a control signal activates the changeover valve to open the connection between the accumulator and the lifting cylinder via the hydraulic machine. The advantage is hereby gained that the stop valves are controlled in an energy-efficient manner to open or close in an optimum manner for the system by means of control signals from an operator or an automated monitoring system.
According to a further aspect according to the invention, it is the case that said cylinder is of the double-acting type comprising a rod side and a cylinder side, the side which is not directly connected to the hydraulic machine being able to receive oil from a hydraulic pump via a control regulator. The advantage is hereby obtained that the handling device""s traditional hydraulic system can be used to supplement regulation of the lowering movement of the lifting device, especially when the variable lowering load is too low to be able to make a positive contribution to the lifting circuit.
According to a further aspect according to the invention, it is the case that the hydraulic circuit comprises a second accumulator, which is connected via at least one nonreturn valve to at least one of the lines between the accumulator and the hydraulic machine or the hydraulic machine and the lifting cylinder. The risk is hereby eliminated of the hydraulic machine xe2x80x9crunning dryxe2x80x9d, i.e. operating without a supply of hydraulic oil. This is namely an evident risk in a system according to the invention, since the oil which is located in the main accumulator is a limited quantity and the flow emitted from the accumulator is terminated instantaneously when this is emptied. As soon as such a hydraulic machine xe2x80x9cruns dryxe2x80x9d, there is a risk that it will seize. This can happen in the course of fractions of seconds. It is thus important that oil can be supplied directly from another part of the system. Normally, the usual hydraulic pump of the handling device will not suffice here, as it usually requires a short start-up period to be able to deliver an adequate oil flow. Thus it is necessary in certain systems according to the invention to provide a second accumulator, which communicates directly via nonreturn valves with the circuit with the hydraulic machine, in order that oil can be supplied instantaneously with the aim of eliminating the risk of damage.
According to further aspects relating to a hydraulic circuit according to the last-named type, comprising a second accumulator, the following holds good:
that said second accumulator is connected via at least one, and preferably two, nonreturn valves to both the line between the accumulator and the motor and the line between the motor and the lifting cylinder.
that the system pressure in said second accumulator is considerably lower than in said first accumulator.
According to a specific aspect according to the invention, it is the case that the hydraulic circuit with the lifting cylinder and the accumulator communicates with a second hydraulic circuit for a slewing turning device, which second circuit comprises valve elements which in connection with deceleration of said stewing part supplies hydraulic fluid to said accumulator, whereupon this is filled and at the same time acts on the slewing part with a decelerating force. Thanks to this solution, a large part of the braking energy from the slewing movement can thus be recovered in the system. In addition, it offers the advantage that the additional energy often appears at an optimum stage, i.e. when the accumulator is on the point of being emptied, as the stewing motion is often operated at the same time as lifting and since the slewing motion is often terminated before the lifting motion is completed. This additional energy thus often comes to the accumulator at precisely the right moment, i.e. when the accumulator is almost empty, providing renewed accumulator power so that the lifting motion can be completed by means of oil which is supplied via the accumulator.
Further aspects and advantages according to the invention will be evident from the more detailed description below.